(1) Field of the Invention
This invention relates to a method of production of cylindrical bodies, and more particularly it is concerned with a method of production of a cylindrical body suitable for use as a yoke of a starter motor or other dynamoelectric machines.
(2) Description of the Prior Art
A starter motor usually used for starting an internal combustion engine is described, for example, in U.S. Pat. No. 3,594,599, in which a yoke in the form of a cylindrical body formed of iron has secured to its inner peripheral surface poles which include permanent magnets and field coils, and an armature core is located adjacent inner peripheral surfaces of the poles. The cylindrical body serving as the yoke has hitherto been produced by one of the following two methods. In one method, a sheet of iron is cut into predetermined lengths which are each rounded into an annular shape and joined by welding at ends abutted against each other to provide cylindrical bodies of a desired size. In the other method, a tubular blank formed of steel is cut into predetermined lengths to provide cylindrical bodies of a desired size. However, these two methods of the prior art are not without disadvantages. The method relying on welding for producing yokes would require a correction of the shape because the cylindrical bodies produced would undergo thermal deformation. Also, a problem would be raised that the influences of heat might cause a reduction in magnetic characteristics, thereby adversely affecting the performance of the motor. When a tubular steel blank is used as material, the problem of high cost would be raised because a solid-drawn steel pipe is high in cost and the tubular steel blank would have to be prepared specially for the purpose, depending on the dimension of the outer diameter.
To obviate the aforesaid disadvantages of the prior art, proposals have been made to use a method wherein a sheet of metal cut in a predetermined size is formed at opposite sides with a protuberance and a groove which are complementary to each other and subjected to rolling to form same into a cylindrical shape, and the protuberance and groove of the rolled sheet are interfitted to provide a cylindrical body of the desired dimensions. Since the protuberance is merely fitted in the groove when the sheet of metal is formed into a cylindrical shape, it is inevitable that gaps are formed between the protuberance and groove when one is fitted in the other. Formation of the gaps causes an increase in reluctance and has adverse effects on the magnetic characteristics.